1. Field of Invention
The invention relates to a method for producing polybutylene terephthlate (PBT) by direct esterification and a catalyst composition therefor, and in particular, to a method comprising esterifying terephthalic acid with 1,4-butane diol followed by polycondensation in the presence of organic titanium compounds as primary catalysts and alkaline metal borates as secondary catalysts to produce PBT.
2. Description of the Prior Art
There are numerous literature references and patents relating to synthesis of polybutylene terephthlate (PBT). Although most of these comprise transesterification processes of dimethyl terephthalate(DMT), but a process of direct esterification of terephthlic acid (TPA) had been developed in 1949 as described in U.S. Pat No. 3,936,421. Since 1970, DMT processes have been commercialized sequentially by Celanese, US and the like, however, since TPA process has an advantage over DMT process in the sense of cost, a trend for synthesis of PBT by changing from DMT process into TPA process became apparent. Since the 1970's, the patent literature reported about producing PBT by TPA processes were mostly focused on the studies of selection of catalysts, composition, feed ratio, reaction mechanism and kinetics.
Synthesis of PBT by TPA process has a most difficult technical bottleneck at cyclisation of butane diol (BDO) into tetrahydrofuran (THF) under acid catalysation, such that, if BDO loss is higher than 0.2 mole per mole of TPA, i.e., at a mole ratio of BDO/TPA of 1.7, the loss of BDO is higher than 11%, then TPA process will be no longer more economical than DMT process, that is why most patents relate to the lowering of THF by product formation.
Generation of THF by cyclisation of BDO is predominantly subjected to acid catalysation. Although the solubility of TPA in BDO is low, however, because of strong acidity of TPA, there is sufficient quantity of acid generated acid to catalyze dehydration of BDO, as described in two routes of mechanism shown in FIG. 1. In the presence of acid (H+), BDO will form a carbonium-ion intermediate with excess electron pair of oxygen atom and then form THF via route 1 or 2 shown in FIG. 1.
As shown in FIG. 1, route 1 is a mechanism of SN1 type, comprising dehydration and then cyclisation to form a cyclic intermediate, and finally, deprotonation into formation of THF, wherein the rate is determined on the bonding rate in the cyclisation of C.sub.1 and C.sub.4 in the primary carbonium-ion, which is a first order reaction. While route 2 is a SN.sub.2 type mechanism, wherein OH.sub.2 at position C.sub.1 is a leaving group, whereas OH at C.sub.4 position is an entering group, and the reaction rate is determined on the rate of leaving of OH2 group as well as entering of H to form cydic product which is a second order reaction.
FIG. 2 is a schematic flowsheet of PBT polymerization, wherein those referred by numerals are activation energies of various reaction. In their study on direct esterification of PBT by utilizing model molecule, Pilati et al. (Polymer 17, 799 and 22, 1566) found that the activation energy for formation of THF from BDO is 30.9 kcal/mole, which is higher than that of step 1 of 22.9, but lower than those of step 3 (33.7) and step 5 (33.7), so that reaction at elevated temperature although increases the rate of efficiency of esterification but it also promotes the cyclisation of BOD, while reaction at low temperature may reduce the cyclisation of BOD, but it also reduces the rate of esterification.
Patents relate to the synthesis of PBT via direct esterification process are as follow:
U.S. Pat. No. 3,936,421: uses organic titanium and tin compounds as catalysts, loss of BOD can be reduced, and it also reports that addition of 2-8% of water can increase mixing and dispersing of terephthalic acid and butane diol which results into increasing of initial esterification rate.
U.S. Pat. No. 4,014,858: uses also a combination of titanium and tin type catalysts in order to obtain a minimum esterification time and hence to generate less amount of THF.
U.S. Pat. No. 4,329,444: uses titanium-based catalyst and preheats butane diol to reaction temperature, whereupon terephthalic acid is added slowly so as to decrease the generation of THF.
U.S. Pat. No. 5,015,759: uses also titanium-based catalyst, mole ratio of BDO:TPA is 5:1, and completes esterification at 225 C. in order to reduce generation of THF.
U.S. Pat. Nos. 4,364,213 and 4,439,597: before terephthalic acid is exhausted or 20-30% thereof left unconverted yet, begins to evacuate and carry out polycondensation reaction. When the input mole ratio of BDO/TPA is low then the esterification rate will be slow, on the other hand, if the mole ratio of BDO/TPA is high, the loss of butane diol will be great, hence the optimum mole ratio is in the range of 1.5-1.8. The temperature of esterification is in the range of 208 C. to 212 C.
U.S. Pat. No. 4,565,241: mole ratio of BDO/TPA at initial reaction stage is 0.6-1.0, addition of BDO takes place at latter stage of reaction.